Giuliani was admitted to Georgetown University Hospital on Sunday, a source familiar confirmed to CNN.
There have been no additional details provided about his condition. It is unclear when Giuliani received a positive test for Covid-19.
Giuliani and his spokeswoman have not responded to CNN’s requests for comment.
The former mayor of New York has been crisscrossing the country to battleground states, leading the President’s long-shot legal battle to overturn the results of the 2020 presidential election.
Giuliani has repeatedly disregarded public health guidelines, appearing maskless in state capitols, hotel ballrooms and at indoor news conferences in recent weeks. At many of the events, most of the other people in the indoor settings were also not wearing masks.
At some of those events, Giuliani was spotted shaking hands, hugging and taking pictures with people while not wearing a mask.
He was most recently at the Georgia Capitol in Atlanta on Thursday to attend a Georgia state Senate hearing on the November election. During a break in the hearing, Giuliani removed the mask he was wearing to greet and take pictures with supporters.
He traveled to Michigan on Wednesday for a state House committee hearing that lasted four and a half hours, during which he was maskless as he pushed misleading claims that the election was stolen from Trump.
At the beginning of the week, Giuliani was in Arizona Monday, meeting with some GOP members of the state’s legislature to discuss unsubstantiated allegations that the election was fraudulent.
The 76-year-old is considered at high risk for complications from the coronavirus due to his age.
Giuliani has been traveling with Trump campaign attorney Jenna Ellis, who has appeared at his side at all of the events about the 2020 election in the last week.
Ellis on Sunday tweeted out “prayers for Mayor @RudyGiuliani” and called him a “tough warrior.” She vowed the President’s legal team will continue its efforts to overturn the election results.
The Trump campaign did not respond to a request for comment about whether Ellis has also tested positive and whether she is quarantining.
Last month, Giuliani’s son, Andrew, who is a White House staffer, tested positive for Covid-19 after attending a Trump campaign legal team news conference with his father in Washington, DC.
Giuliani also eschewed US Centers for Disease Control and Prevention quarantine guidelines after his son, and close contact and campaign adviser Boris Epshteyn, tested positive in late November. The day Epshteyn announced he had tested positive, Giuliani traveled to Gettysburg, Pennsylvania, where he addressed a group of Republican state lawmakers in a crowded hotel ballroom.
The former mayor is the latest person in the President’s orbit to contract the virus. The President, first lady Melania Trump, his sons Donald Jr. and Barron, his chief of staff Mark Meadows and a number of other top aides both in his campaign and in the White House have tested positive in recent months.
Giuliani was also in close proximity with Bill White, a Trump booster in Georgia who attended the President’s rally Saturday night in Valdosta. White shook hands and closely embraced Giuliani last week.
As the number of Covid-19 cases and deaths have been on the rise, the White House has continued to flout CDC recommendations and host large events without masks and little social distancing.
This story has been updated with additional reporting Sunday.
CNN’s Annie Grayer, Wesley Bruer and DJ Judd contributed to this report.